Heat-shrinkable plastic films are widely used for production of heat-shrinkable packages, heat-shrinkable labels, and the like by utilizing the heat-shrinking property thereof. Among such films, drawn films such as polyvinyl chloride film, polystyrene film, and polyester film are used for production of labels, cap seals, or refuse (garbage) packages in various kinds of containers such as polyethylene terephthalate (PET) containers, polyethylene containers, and glass containers.
However, polyvinyl chloride films have low heat resistance, and likely to emit hydrogen chloride gas at the time of incineration, thereby emitting dioxin. Further, in use of heat-shrinkable vinyl chloride resinous film as shrinkable labels on PET containers or the like, the labels and the containers are required to be separated from each other in recycling of the containers.
Polystyrene films are preferable in the aspect of good finishing appearance after shrinkage. However, since they have low resistance to a solvent, it is required to use an ink having a special composition in printing. Further, polystyrene resin is required to be incinerated at a high temperature, and a large amount of black smoke and nasty odor are likely to be generated at the time of incineration.
There is a great expectation for polyester films free of such problems as substitute shrinkable labels for polyvinyl chloride films and polystyrene films. As consumption of PET containers is increased, there is a tendency that consumption of polyester films is increased.
Despite of the above merits, there is a demand for further improvement in the conventional heat-shrinkable polyester films in the aspect of shrinking property. Particularly, characters or patterns that have been printed on films before shrinkage are likely to be distorted at the time of attaching the films to containers such as PET bottles, polyethylene bottles, and glass bottles by heat shrinkage owing to generation of shrinkage shading, or crinkling. Accordingly, there is a demand from users of reducing such distortion.
Compared with heat-shrinkable polystyrene films, polyester films have poor shrinkage at a low temperature. In view of this, it is required to shrink polyester films at a high temperature to obtain a required shrinkage, which may likely to cause deformation of bottle bodies and whitening of bottles.
In attaching heat-shrinkable films to containers by heat shrinkage, the films are shaped into various shapes such as labels and bags after a printing step, if necessary. After placing of the label-like films or the bag-like films onto the containers, the films are tightly attached to the containers by heat shrinkage while the containers are conveyed on a belt conveyor or the like through a shrinking tunnel (steam tunnel) wherein heated steam is blown in for heat shrinkage or a shrinking tunnel (hot-air tunnel) wherein hot air is blown in for heat shrinkage.
A steam tunnel provides higher heat transfer efficiency than a hot-air tunnel and makes it possible to perform more uniform heat shrinkage. Compared with a hot-air tunnel, a steam tunnel provides superior finishing appearance after heat shrinkage. However, compared with polyvinyl chloride films and polystyrene films, the conventional heat-shrinkable polyester films are poor in shrink finishing appearance after being conveyed through the steam tunnel.
Further, in use of a hot-air tunnel in which temperature spots are likely to appear during heat shrinkage, whitening, shrinkage shading, crinkling, distortion or a like drawback is likely to appear on the conventional polyester films. Particularly, whitening causes a serious deterioration in product appearance. The conventional polyester films are inferior to polyvinyl chloride films and polystyrene films in shrinkage finishing after being conveyed through a hot-air tunnel.
Furthermore, if drawing is increased in an attempt to secure a high shrinkage factor, the film is likely to be torn in a direction orthogonal to the shrinkage direction. Thereby, tear-related troubles of the film may likely to occur during a printing step, a label producing step, or after shrinkage. An improvement for such troubles has been demanded.
In view of the above, it is an object of the present invention to solve the problems residing in the conventional heat-shrinkable polyester films, and to provide heat-shrinkable polyester films suitable for production of labels which have an excellent shrinking property in a wide temperature zone from a low temperature to a high temperature, as well as excellent tear resistance, and solvent adhesiveness, and have no or less whitening, shrinkage shading, crinkling, distortion, uneven shrinkage in a direction orthogonal to the maximum shrinkage direction, or the like.